The present invention relates generally to methods for producing rolled, annealed steel strip and more particularly to methods for producing cold rolled steel strip containing elements which undergo internal oxidation during annealing.
Cold rolled steel strip is produced from a relatively thick cast steel article which is subjected to a series of hot rolling steps, during which the steel article is at an elevated temperature and undergoes successive reductions in thickness to produce a relatively thin hot rolled strip which is coiled or collected, cooled to room temperature and then subjected to a cold rolling operation, conducted at room temperature, during which the steel is reduced to substantially its final thickness. Cold rolling imparts to the steel certain physical properties, such as increased hardness and strength and decreased ductility.
During production of the cold rolled steel strip, the strip conventionally undergoes an annealing operation, either between hot rolling and cold rolling, between stages of the cold rolling operation, after cold rolling or a combination thereof. Annealing is conducted at an elevated temperature (e.g., 1250.degree.-1550.degree. F. (682.degree.-843.degree. C.)), and it affects the physical properties of the steel and the ease with which a steel strip can undergo further deformation or fabrication.
Cold rolled steel strip is the basic material from which many steel parts are fabricated. In some instances where it is desirable for the fabricated part to have a very low carbon content (e.g., when the fabricated part is used in the core of an electric motor or of a transformer), a decarburizing process is conducted in conjunction with the annealing of the cold rolled strip.
Most steels contain, in addition to iron and carbon, other alloying elements for imparting to the steel certain specific properties. Some of these additional alloying elements have an affinity for oxygen greater than that of iron. For example, in an electrical steel used as the material for the core of an electric motor, silicon and aluminum are added to improve the properties of the electrical steel. Both silicon and aluminum have an affinity for oxygen greater than that of iron and, indeed, greater even than that of carbon. When these alloying elements are uncombined in the steel and the steel is subjected to an annealing operation under oxidizing conditions, the uncombined alloying elements will undergo oxidation and form an internal oxidation layer adjacent the surface of the steel product. When this occurs, the alloying elements are unavailable to perform the function for which they were added to the steel, and the properties of the steel suffer. In addition, the internal oxidation layer itself has an adverse effect on the magnetic properties of an electrical steel containing silicon and aluminum, and the adverse effect increases with an increase in the thickness of the internal oxidation layer.
An internal oxidation layer of the type described in the preceding paragraph can also form during heating incident to hot rolling, but because the steel strip is relatively thick during hot rolling, the thickness of the oxidation layer is relatively small as a percent of the strip's total thickness, and the amount of alloying element undergoing oxidation is relatively insignificant from the standpoint of the diminution of the properties of the steel for which the alloying element was added. Only when a steel strip approaches or is at its final thickness, does the thickness of the oxidation layer become significant. This condition exists after the strip has been cold rolled, either with or without temper rolling.
The more oxidizing the atmosphere in which the steel strip is heated, the more favorable are the conditions for forming an internal oxidation layer adjacent the surface of the steel product. When a steel strip is subjected to a decarburizing anneal, the conditions are very favorable to the formation of an oxidation layer adjacent the surface of the steel strip.